Etymology
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limb (n.1)

"part or member," Old English lim "limb of the body; any part of an animal body, distinct from the head and trunk;" main branch of a tree," from Proto-Germanic *limu- (source also of Old Norse limr "limb," lim "small branch of a tree"), a variant of *lithu- (source of Old English liþ, Old Frisian lith, Old Norse liðr, Gothic liþus "a limb;" and with prefix ga-, source of German Glied "limb, member").

The unetymological -b began to appear late 1500s for no etymological reason (perhaps by influence of limb (n.2)). The Old English plural was often limu; limen and other plural forms in -n lasted into Middle English. Since c. 1400 especially of a leg; in Victorian English this usage was somewhat euphemistic, "out of affected or prudish unwillingness to use the word leg" [Century Dictionary]. However in Old and Middle English, and until lately in dialects, it could mean "any visible body part":

The lymmes of generacion were shewed manyfestly. [Caxton, "The subtyl historyes and fables of Esope, Auyan, Alfonce, and Poge," 1484]

Hence, limb-lifter "fornicator" (1570s). Limb of the law was 18c. derisive slang for a lawyer or police officer. To go out on a limb in figurative sense "enter a risky situation" is from 1897. Alliterative life and limb in reference to the body inclusively is from c. 1200. Obsolete limb-meal (adv.) "limb-from-limb, piecemeal" is from late Old English lim-mælum.

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limb (n.2)
late 14c., "edge of a quadrant or other instrument," from Latin limbus "ornamental border, hem, fringe, edge," a word of uncertain origin. Klein suggests it is cognate with Sanskrit lambate "hang down limply" and English limp (adj.). Tucker writes that "the sense appears to be that of something which twists, goes round, or binds ... not of something which hangs loose," and suggests cognates in Lithuanian linta "ribbon," Old Norse linnr "whether." De Vaan tends to agree with Klein and writes, "In view of the phoneme *b, the very specific meaning of limbus and its absence from the oldest literature, the etymology remains uncertain." Astronomical sense of "edge of the disk of a heavenly body" first attested 1670s. Related: Limbal.
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limbless (adj.)
1590s, from limb (n.1) + -less. Related: Limblessness.
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limbus (n.)
Latin, literally "edge, border" (see limb (n.2)). Used in English in various senses; in Medieval Latin the name of the region on the border of Hell, and thus sometimes used in very correct English for limbo (n.1).
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limbate (adj.)
"edged, bordered," in botany, of flowers in which one color is edged by another, 1826, from Late Latin limbatus, from Latin limbus "border, hem, fringe, edge" (see limb (n.2)). Related: Limbation.
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limbic (adj.)
"pertaining to or characteristic of a border," 1879, in anatomy, in reference to the brain, from French limbique (1878, Broca), from limbe (14c.), from Latin limbus "edge" (see limb (n.2)). Limbic system is attested from 1950.
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limber (n.)
"detachable forepart of a field-gun carriage," 1620s, alteration of Middle English lymer (early 15c.), earlier lymon (c. 1400), probably from Old French limon "shaft," a word perhaps of Celtic origin, or possibly from Germanic and related to limb (n.1). Compare related Spanish limon "shaft," leman "helmsman."

The nautical limber "hole cut in floor timbers to allow water to drain" (1620s), however, appears to be unrelated; perhaps from French lumière "hole, perforation," literally "light."
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numb (adj.)

c. 1400, nome, "deprived of motion or feeling, powerless to feel or act," literally "taken, seized," from past participle of nimen "to take, seize," from Old English niman "to take, catch, grasp" (from PIE root *nem- "assign, allot; take"). The unetymological -b (to conform to comb, limb, etc.) appeared 17c. The notion is of being "taken" with palsy, shock, and especially cold. Figurative use is from 1560s.

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limber (adj.)
"pliant, flexible," 1560s, of uncertain origin, possibly from limb (n.1) on notion of supple boughs of a tree [Barnhart], or from limp (adj.) "flaccid" [Skeat], or somehow from Middle English lymer "shaft of a cart" (see limber (n.)), but the late appearance of the -b- in that word argues against it. Related: Limberness. Dryden used limber-ham (see ham (n.1) in the "joint" sense) as a name for a character "perswaded by what is last said to him, and changing next word."
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limbo (n.1)
region supposed to exist on the border of Hell, reserved for pre-Christian saints (Limbus patrum) and unbaptized infants (Limbus infantum);" c. 1300, from Latin limbo, ablative singular of limbus "edge, border" (see limb (n.2)). In frequent use in Latin phrases such as in limbo (patrum), which is entirely Latin, but the in was taken as English and hence the Latin ablative became the English noun. Figurative sense of "condition of neglect or oblivion, place of confinement" is from 1640s.
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